1. Field of the Invention
The present invention relates to a method and device to measure the lithium concentration in the primary cooling circuit of a pressurized water reactor of a nuclear power station.
2. Description of the Prior Art
In the cooling water of a reactor of nuclear power station of the pressurized-water reactor type, boron must be introduced in the form of boric acid, in order to regulate the flow of neutrons. This cooling circuit is called the primary circuit.
During a production cycle, i.e. during the operation of the reactor between two fuel reloadings, the boron concentration varies from about 2,000 ppm or parts per million to almost zero ppm.
In the cooling circuit, the corrosion products become radioactive after they have travelled through the reactor core. These products are partly responsible for the radiation undergone by operating and maintenance staff in nuclear power stations.
Studies conducted, using the PACTOLE computation code drawn up by the French Atomic Energy Commission (Commissariat l'Energie Atomique) have shown that, by maintaining a constant pH in the water of the primary circuit, it is possible to substantially reduce the quantity of these radioactive products and, consequently, the intensity of the radioactive radiation and the dose absorbed by maintenance staff. To this end, it is therefore advantageous to neutralize the boric acid partially by injecting a base, such as lithium hydroxide (LioH) into the cooling primary circuit.
Thus, FIG. 1 shows the relationship governing the variation of the lithium hydroxide and boron concentration in the primary cooling circuit to keep a constant pH during a production cycle.
In order to comply with the relationship governing the variation of the lithium hydroxide concentration, as shown in FIG. 1, a variation relationship known as the decreasing lithium hydroxide specification, the lithium concentration in the primary cooling circuit has to be measured. For this purpose, a sample of cooling water is taken and the lithium concentration is measured by atomic absorption spectrometry. If the lithium hydroxide concentration is not satisfactory, it must be adjusted in the circuit and another measurement must be made to ascertain that the new concentration measured corresponds the above-mentioned decreasing lithium hydroxide specification. All the above-mentioned operations require lengthy stages for their application and, moreover, the adjusting of the lithium hydroxide concentration is not easy to do during the measurement inasmuch as even if the quantitative proportioning of lithium hydroxide is achieved with sufficient precision, it can hardly be ensured in any valid way without taking into account the response time of the entire primary cooling circuit to reach the desired pH value.